CORE: A Study of OPC-6535 With Asacol® in Maintaining Ulcerative Colitis (UC) Remission

Objective(s): This study will compare the safety and efficacy of 25 mg per day (QD) and 50 mg QD of OPC-6535 to 800 mg twice a day (BID) of Asacol® in the maintenance of remission in subjects with ulcerative colitis. Subject Population: * Subjects with ulcerative colitis currently in remission defined as rectal bleeding (RB) and flexible sigmoidoscopy (FS) scores of 0, on or off a stable dose of sulfasalazine or 5-ASA products for at least 6 weeks. * Subjects must have had the diagnosis of ulcerative colitis established by prior colonoscopy or undergo colonoscopy in lieu of flexible sigmoidoscopy during the Screening Period. * Subjects must have had treatment for a flare of ulcerative colitis, with symptomatic onset of remission occurring no more than 52 weeks from the Screening Period. * Subjects may not have used corticosteroids, topical agents (corticosteroid or 5-ASA enemas, suppositories, foams), azathioprine, 6-mercaptopurine, or methotrexate within 6 weeks of the Screening Period. Upon entry, sulfasalazine and 5-ASA containing products will be discontinued. Safety: Vital signs, ECGs, laboratory studies (including hematology, clinical chemistry, and urinalysis), and adverse events.